It doesn't make any sense: all I get is a vaguely trapiezoidal wave at ~200KHz. Panel controls don't make any difference.

Sorry about the DesignSpark schematics - I've tried to make them as readable as possible. Panel controls are on a separate board and are not shown, but they closely follow the Ian Fritz schematic with the addition of an extra exponential FM control+socket.

There was one on the original Electronotes schematic. Bernie Hutchins hinted that it might no be necessary if you used an LF351 which is what I actually used, but I think you might be right. The first schematic has a 39pF cap as compensation.

Junk the expo circuit and mock up a simple linear one instead to first get the integrator working, then re-introduce the expo. Don't try and solve too many variables.Keep looking.Use a continuity tester to check joints.

Too many variables, but I'm determined to get a reliable design together. I'll try a linear current source with it. I've been going down the road of gradually splitting everything into it's component parts, which is why I'm only building the core of the VCO here.

JFETS - they do seem like a poisoned chalice in terms of reliability and specification. I am thinking that maybe a fast analogue switch like 1/4 of a CD4066 would be better.

Are you building this particular VCO for any reason? If not, I can suggest an easier one.

If yes,

How many volts are you supplying it?

First thing, remove the expo converter circuit so you can just troubleshoot the oscillator. Connect a pot biased between +V and -V to the input. Tweak the pot until you get the slowest possible oscillation.

Try increasing the value of the capacitor (C2) between the comparators input and output to ~100pf. If this makes no difference then...

Try using a larger cap in the integrator. increase 2.2n to 10n. If this makes no difference...

Try a Tl071 instead of a LM311 for the comparator. You will definitely need C2 to be 100pf for this. If its still too fast increase C2 to a higher value.

Replace the voltage divider on pin 3 of the comparator with a pot so you can adjust the level at which the oscillator resets.

Re: the JFET. You can also try a BF245.Last edited by jackdamery on Mon Sep 30, 2013 4:51 am; edited 3 times in total

It probably won't work with a TL071 for the integrator. It's a tight design, and you cannot just make arbitrary component substitutions.

Ian

Do you mean the comparator? I subbed the TL080 in the Jupiter 8 - a similar circuit for a TL071 and it performed just fine.

synchroma can you point your oscilloscope at pin 7 of the LM311 so we can see what the comparator is doing. I suspect the pulse width is too long and we are seeing the integrator charge to full (the ramp) then wait to be reset (the flat line) then discharge slowly (the falling ramp).

OK. I'm beginning to realise how critical the choice of op amp is for many of the designs I find out there! Apologies for using the "TL071" symbol in my schematic - it was a means to an end to get a working symbol/PCB component. I realised I had a fairly bogus Eagle op-amp library I imported into my CAD package with the wrong pinouts, and had to go with the crappy downloadable part spec from the software makers :-/

So, the TL071 is actually an LF351 in the circuit. Forgot to mention it, sorry! I was going with what Bernie Hutchins wrote that "You can also use an LF351 for the CA3140. You can also use an LF351 for the CA3130, but it would be run on the bipolar supply, and without the 39p compensating capacitor." (source: http://www.synthsource.com/ens76/vco4.htm )
It seems that this is incorrect. It didn't work.

So...I've bought a load of components: CA3130 and CA3140, OPA2132 (can't seem to get the OPA132 in the UK) LM311 and LM319 comparators, LM329DZ precision regulator and a bunch of other tightly spec-ed components (2N4391, 2N4859, 60K4 resistor etc). And, I've made a board with three different saw cores on it which (if my kids let me) I'll solder and test over the weekend.

Hutchins / Mikulic (ENS 76)
Rene Schmitz
Ian Fritz (with the other half of the LM319 used for PWM)

I still have the expos I made from Ian Fritz's design so I'll plug them in once I know the saw cores work (I'll knock up a linear current source to test them first).
Cheers

saw_board.JPG

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Only top copper is shown. If they all work as intended, and if there is some interest, I'll make the design available here.

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Last edited by synchroma on Sun Oct 06, 2013 11:26 am; edited 1 time in total

I notice that threads go cold on this forum after the first post from everyone. So, I'll start a new thread with the results I have from my build, which is less than encouraging as far as me working this out is concerned!

If only Thomas Henry had an integrator+reset design. His designs actually work.

I have now etched an built 5 sawtooth VCO cores without a single working oscillator. Mighty miffed.

Here is the board, both the artwork and a superimposed component view. It's not very well labelled I'm afraid, but I've taken as much care as possible to follow the schematics I found online. You can view the full 600dpi from the zip file.

A few changes - the ENS76 Option 1 VCO has two voltage followers providing 2 outputs (LF353 to the right of the board) and I've put together a PWM section from the other half of the LM319 in Ian Fritz's schematic.

Does anyone else know if there's a decent way to test an expo converter by itself? That's the other place I'd be looking, but I'm not sure whether running a resistor to ground would let you check the output or would just load it down.

It does indeed have the compensating cap. And it is run at +15V and ground, as in the original. See the image below...After the first failed attempt I remade the board with exact matches for all components - I couldn't find a 39pF cap though - I used 33pF. Perhaps I should parallel a 10pF cap with it to make 43pF? I'll try this later when I get back to the lab.

I'd also like to find out how to test an expo on its own. I would have thought that running a resistor to ground would do it though, since expos are usually designed to produce changes in current. So loading an expo designed to peak at 1mA with a 1K resistor should produce a voltage that goes up to 1V, since V = IR

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